This invention relates to fiber optic wavelength division multiplexers/demultiplexers and more particularly to such multiplexers/demultiplexers which provide total or partial redundancy.
One of the distinct advantages of fiber optics is the ability to carry a large number of channels at different optical wavelengths. With each channel capable of transmission at rates up to 400 megabits or greater, the multiplexing of many wavelengths onto a single fiber would increase the data capacity enormously.
A known method of wavelength division multiplexing/demultiplexing uses a gradient index of refraction or GRIN lens in which the index of refraction varies from the center out along a radius. Such a multiplexer/demultiplexer device using a GRIN rod is described by W. J. Tomlinson and G. D. Aumiller in Applied Physics Letters, Vol. 31, No. 3, Aug. 1, 1977, pages 169-171. A blazed diffraction grating serving as the optically dispersive element is placed near the end of the GRIN lens and oriented at an angle to it.
When used as a demultiplexer the GRIN lens/grating combination has optical energy entering the GRIN lens through an input optical fiber. Several wavelengths will have been combined to travel along the fiber and each of the wavelengths will have information impressed upon it. At the receiving end, the multiplexed wavelengths must be separated so that the information on each of the signals can be extracted. For example, if four separate wavelengths have been multiplexed and are traveling down the input fiber, the ray will travel through the GRIN lens and strike the difraction rating. The grating will diffract and reflect the beam while at the same time dispersing it into four beams. The four beams will travel back through the GRIN lens and exit at output locations at the surface of the lens. Output optical fibers are appropriately located to intercept each of the beams for subsequent signal extraction.
In many applications, it is critical to provide redundancy to minimize disruption in the communication link. One type of redundancy is path redundancy which can be accomplished by using more than one fiber extending from the source to the receiver. In such a case, if the primary input fiber fails for some reason, information can be switched over into the secondary fiber so that communication can continue. Another type of failure is failure of the terminal equipment. In this case, after signals have been demultiplexed, it may happen that one or more of the detectors used to extract information from the demultiplexed signals has failed. In such a case, it is important to provide back up terminal equipment which can be utilized when the primary terminal equipment ceases to function properly.
It is therefore an object of this invention to provide a multiplexer/demultiplexer which can provide path redundancy, terminal equipment redundancy or both path and terminal equipment redundancy.
It is a further object of this invention to provide such a multiplexer/demultiplexer with total or partial redundancy which is highly reliable and simple in construction.